This invention relates to photonic couplers and methods of making the same, and more particularly to such components and methods used to fabricate reduced diameter, photosensitive optical fibers to and record grating patterns therein.
Modern communication systems are increasingly based on optical transmission through optical fibers, because of the superior bandwidth capabilities of optical signals and the fact that a single optical fiber can transmit many different channels, as by wavelength division multiplexing. To realize the potential of such systems, wavelength selective devices, including couplers and filters, have been recently developed to meet the requisite design and performance specifications. These requirements include precise wavelength selectivity, low crosstalk, flat passbands, low dispersion and low insertion loss. These are all necessary to avoid diminution of signal strength and the introduction of signal distortion, as devices are cascaded to perform various multiplexing and demultiplexing functions.
Many wavelength selective components for these purposes are based upon the approach of embedding or writing a periodic pattern, such as a Bragg grating, in an optical fiber, so as to reflect or transmit only a very narrow wavelength band within a much broader spectral range, for example, the entire C or L WDM band. One example is a four terminal add/drop coupler formed from two optical fibers merged at an intermediate region and incorporating a Bragg grating. A substantial departure from prior concepts that use this basic configuration is described in U.S. Pat. No. 5,805,751 to Kewitsch et al., entitled xe2x80x9cWavelength Selective Optical Couplersxe2x80x9d, and assigned to the assignee of the present invention. Devices as taught in this patent are grating assisted and typically asymmetric. They operate with high efficiency in typically a reflective mode or alternately in a transmission mode. They are further characterized by a non-evanescent, very small diameter coupling region in which two optical fibers are longitudinally fused. In this coupling or waist region, signals are guided in a glass-air waveguide mode, because the original cladding is now of small diameter and the doped cores of the fibers have been reduced to vestigial elements which have only a small effect on waveguiding. After the fibers are narrowed and merged, a periodic index of refraction pattern Bragg grating) is written in the small diameter coupling region, which is typically less than about 10 microns in cross-sectional dimensions but is photosensitive because of its dopant content, the use of in-diffusion of a photosensitizing gas, or both.
The process used to form a merged coupling region presents some unique problems involving multiple disciplines that extend well beyond the present day techniques used to produce fused splitters. For example, to illuminate the coupling region with uv light through a mask so as to record a grating pattern, the target material must remain photosensitive. However, the very small diameter coupling region must be formed by controlled elongation and fusion as the optical fiber is heated to the softening point, a process that can significantly affect the photosensitivity of the glass. To maintain low loss and control of elongation, the heating is generally best done with a reciprocating flame, recognizing that the temperature of the flame as well as the chemical composition of the heating gas can influence the subsequent photosensitivity. Furthermore, because the fibers in the coupling region are of micron range diametral size, the fibers cannot withstand the force of a flame of substantial velocity without deflecting and/or deforming. Moreover, the strength of the grating that is ultimately written is dependent on all stages of the process, from initial photosensitivity of the starting fiber cladding material, through heating and drawing, to the completion of an exposure step. The interrelationships of these factors have not heretofore been fully understood or utilized, but it is clear that improvements can be made in grating efficiency, passband characteristics and in product yields as well.
While achieving a photochemical state in which photosensitizing potential is brought to a high level is more than adequate in and of itself for many purposes, more is increasingly being required of photonic devices using index of refraction patterns. For example, workers in the art are now extending systems and devices toward 25 GHz and 50 GHz applications, thus requiring narrow bandwidth gratings in fibers and couplers. Higher performance is also being sought in add/drop devices for more general use. To meet the increasingly stringent requirements of the modern era, spatial variations in the effective index of variation change (chirp) must be very small, approximately a factor of 10 less than the desired DWDM periodicity. In numerical terms 100 GHz filters require a chirp of less than 0.08 nm, which equates to 0.0008 uniformity in the index of refraction change. For 25 GHz filters the chirp and uniformity of index of refraction change must be 4 times tighter.
Maintaining adequately low crosstalk ( less than xe2x88x9225 dB) further demands that the spatial variation of the index of refraction be extremely smooth along the grating length. Specifically, and superfluous periodicities (ripple) in the grating of between 0.5 microns to 1 mm must be removed to a level better than 5%. The problems of meeting such requirements are compounded when one considers that the exposure response of the photosensitive material varies non-linearly with exposure time, and in a variable manner dependent on the photochemistry of the material. In addition the photosensitivity of the target material varies non-linearly as a function of laser intensity, and the intensity of a beam projected through a varying (i.e. apodized) phase mask also is dependent on position relative to the phase mask.
Systems and methods in accordance with the invention include the use of photosensitizing dopants in a precursor element, such as an optical fiber, heating the fiber during drawing with a diffuse and distributed low hydrogen content flame of very low velocity and of controlled temperature. As the fiber is tensioned, it is locally heated in a repetitive manner by reciprocating movement of the flame until it is drawn down to a selected length of substantially uniform diameter. In illuminating this target region to write a periodic grating, the intensity of the actinic radiation is varied in controlled fashion as a photosensitizing gas is diffused into the fiber, preferably at elevated pressure. The index of refraction change in the target may be further enhanced by optimizing grating growth through balancing of light source intensity, scan velocity, and blue light luminescence from the target fiber.
In more specific examples of systems, devices and methods in accordance with the invention, the target region of a photonic device, i.e. an optical fiber or fibers in which a grating is to be written, includes a constituent (dopant) providing photosensitivity to uv illumination. This region is gently heated with a low velocity, inverted reciprocating flame that locally surrounds the target area of optical fiber. The flame is preferably a mixture of CO and O2, with an inert gas assuring that OH and water by-products will be minimized. Relative humidity and temperature of the surrounding air atmosphere are maintained within selected limits. Flame temperature can be reduced by mixing with an inert gas (such as N2), the amount of which can be adjusted to maintain a desired temperature. After the heated fiber is adequately elongated, the photochemical characteristics of dopants within the fiber, together with the exposure process, determine the grating growth characteristics. By subjecting the fiber during actinic illumination to indiffusion of high pressure deuterium or hydrogen (possibly heating the fiber at the same time) and by maintaining the uv illumination intensity above a selected threshold, the photo-induced index changes contribute to achieving an extremely strong grating. Furthermore, varying the polarization of the uv writing beam during exposure may optimally utilize the photosensitive dopant sites within the glass.
A feature of the invention is the provision of a torch of ceramic material including a diffuser of compressed porous material at its outlet. Pore sizes in the diffuser range from 30 to 100 microns, and the orifice area of the diffuser is about 3xc3x976 mm in area, providing a distributed volumetric flame of low velocity that is at least initially in stoichiometric balance, or alternatively oxygen rich to a degree. Since CO is one constituent, care is taken to ensure against contamination by iron impurities. The flame is maintained at about 2000xc2x0 C. and is of a visible color which allows the fiber location relative to the flame to be precisely determined. The diffuser has the advantageous property that it stabilizes the flame characteristics to reduce thermal fluctuations, which improves the uniformity of the fabricated coupler. Since each coupler is elongated to the same length at the same rate (a characteristic of the manufacturing process that is unique to the asymmetric coupler described here, and is not the case for 50/50 couplers, for instance), multiple fibers may be drawn at the same time. An array of such torches can be used in combination to provide a multiple coupler fabrication station.
Other features in accordance with the invention contribute to the achievement of diametral uniformity in the waist region, and to improved grating strength. The fiber is advantageously held in the flame volume in a region in which the combustible constituents are in approximately stoichiometric proportions, and at or close to maximum temperature, reducing sensitivity to variations. By velocity modulation of the reciprocating scanning motion along the fiber, in which the flame is at a lower velocity in the central region of the scan, then accelerates to a higher velocity until it decelerates and accelerates rapidly to reverse at end points, a short waist region of very uniform diameter is formed in which the grating can be written.
Other methods in accordance with the invention enable realization of the potential imparted by the disclosed photochemistry concepts. Index of refraction gratings for narrow passband add/drop devices and filters having very low crosstalk are achieved by modulation of beam residence time while scanning a photosensitive target element through a selected pattern. In accordance with one example, a photosensitized coupler is scanned repeatedly and unidirectionally, in time separated fashion, by a laser beam whose scanning velocity is varied relative to the length of the grating that is being imprinted. Exposure is ramped up rapidly to a scan start point, varied from a nominal level as the beam travels along a photomask which defines the pattern to be recorded and then ramped down, so that exposures at the terminii of the grating merge smoothly and have reduced crosstalk and backreflection effects. Between these end regions in this example, velocity and therefore exposure, is controlled by sensing photoluminescence at a selected wavelength from the element, and using the sensed signal to provide a constant average index of refraction by compensating for variations caused by the photomask pattern, and also short term variations arising from local non-uniformities. In the successive passes until a desired final result is achieved the modulation minimizes effects from non-linear factors such as sensitivity characteristics and response to laser intensity. Consequently narrow gratings having small chirp, minimized spatial variations and low cross-talk have been provided.